1. Field of the Invention This invention relates to an ejector pin with a pressure sensor that can detect the pressure from a resin filled within an injection mold cavity.
2. Description of the Related Art
FIG. 6 is an enlarged cross sectional view illustrating a metal mold for a conventional injection tool. The metal mold is formed of a load cell 111 which directly detects the pressure in a cavity 110 as well as a load cell 113 which is in contact with an ejector pin 112 extruding a molded component out of the cavity 110, but separately from the ejector pin 112. The load cells 111 and 113 are mounted by machining special holes and sleeves in the metal mold.
In the injection tool shown in FIG. 6, large load cells 111 and 113 cannot be mounted without machining the metal mold. Since being impossible to be machined, some metal molds cannot be mounted. Furthermore, since the mold machining wages, machining labor, and costs of the load cell itself are costly, it is difficult to adopt and widely use the present structure.
To solve such problems, the present applicant has proposed an ejector pin with a novel pressure sensor in which an ejector pin and a pressure sensor are integrated, instead of detecting the cavity pressure with an ejector pin and load cells. In the ejector pin 200 with a pressure sensor as shown in FIG. 7, the strain producing portion 201 with an U-shaped cross section is securely fixed on the base end of the ejector pin 200. A pressure sensor (not shown) is attached on the lower surface of the strain producing portion 201. The strain producing portion 201 is housed in the casing 202.
When the pressure in the cavity of the metal mold is added to the end of the ejector pin 200, the load according to the pressure receive area of the end occurs on the ejector pin 200. The sensor detects the deformation of the strain producing portion 201 due to the load. The pressure in the cavity is detected based on the output from the sensor by referring to previously prepared data representing the relationships between the output from the sensor and the pressure in the cavity.
In the ejector pin with the pressure sensor, a pressure sensor is integrally attached on the back end of the ejector pin, so that the effect can be expected of eliminating the step of machining a metal mold used to attach the pressure sensor.
In order to use an ejector pin, it is needed to suitably set the number of ejector pins and the thickness of an ejector pin according to conditions including the structure of a metal mold, the position at which an ejector pin goes against a molded component, and the shape and thickness of a molded component. Generally, where a relatively great number of ejector pins are disposed, the diameter thereof become relatively small. Where a small number of ejector pins are disposed, the diameter thereof becomes relatively large. Moreover, when a large force to peel off from the metal mold is needed due to differences in position where an ejector pin pushes against a molded component, thick ejector pins are needed. Thin ejector pins may be used when a relatively small force is sufficient for peeling.
However, the ejector pin with a pressure sensor proposed by the present applicant has the whole end surface of the ejector pin acting as a pressure receive surface. Even if the pressure of the cavity of the metal mold is the same, the pressure receive area depends on the diameter of the ejector pin, thus affecting the load applied to the ejector pin. Even if the pressure in the cavity is the same, the output of the pressure sensor varies by changing the thickness of the ejector pin under various conditions previously described. Hence, outputs from the sensor must be amended to agree with reference data showing the relationships between pressures in the cavity and sensor outputs. The following Table 1 shows the corresponding relationships between pin diameters (diameters) of an ejector pin, pressures in a cavity (resin pressures in a cavity), and loads applied on an ejector pin.
For example. as shown Table 1, when the pressure in the metal mold is 500 kgf/cm2, the load on a 1 mmf ejector pin is 3.93 kgf while the load on a 6 mmf ejector pin is 141.30 kgf. Since the sensor output depends on the pin diameter even at the same mold pressure, correction thereof is required. As described above, the ejector pin with a pressure sensor proposed by the present applicant can solve the conventional long-waited problem. However, this type ejector pin requires mathematically processing the sensor output based on coefficients according to the pin diameter. The strain producing portion may be designed for each pin diameter by considering a difference in load due to a difference in pin diameter (different pressure receive area). However, this approach results in increased fabrication costs. When a pin with a large diameter causes a large load, the strain producing portion becomes large, thus exceeding the allowable space for suitable design. As a result, this ejector pin is not sufficiently practical.
Injector pins with large diameter are essential to deal with various release resistances without giving a pressure to a molded component. It has been difficult to the ejector pin with a pressure sensor previously proposed by the present applicant has a large diameter. Hence, the ejector pin with a pressure sensor is used only as the function of a pressure sensor by disposing it as a lightening pin at another place not requiring an ejector pin.
The present invention is made to overcome the above-mentioned problems. The object of the invention is to provide an ejector pin with a pressure sensor that has a fixed pressure receive area (or load) even in different diameters thereof, so that the sensor output can be utilized without any change and no design changes are required according to the pressure receive area.
According to the present invention, in an ejector pin with a pressure sensor, wherein a resin is injected into a cavity formed between a female mold and male mold, the female mold and the male mold forming an injection mold, to mold a molded component, and said molded component is ejected with the end of the ejector pin when the male mold is relatively separated from the female mold, the ejector pin comprises a bar portion to which the pressure of an injected resin is applied; pressure sensing means fixed on a base end of the bar portion, for detecting a pressure applied to the bar portion; and a sleeve portion housing the bar portion and the pressure sensing means.
In the ejector pin according to the present invention, the pressure sensing means comprises a strain producing portion fixed on the base end of the bar portion and elastically deformed by a load applied to the bar portion and a sensor for detecting deformation occurring in the strain producing portion.
Moreover, in the ejector pin according to the present invention, the sleeve portion comprises a guide portion for axially and slidably guiding the bar portion, the end of the bar portion having an opening facing the cavity, and a housing room for housing the strain producing portion.
In the ejector pin according to the present invention, the end of the bar portion is substantially flush with the end of the guide portion in the sleeve portion.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.